Increasingly, various monitoring and processing systems use body monitors. Body monitors are electronic devices which attach to a person, animal, or inanimate object, hereinafter referred to as a body, and perform some function with respect to that body. Numerous examples of body monitors are known. In connection with correctional facilities, prisoners may wear body monitors that, among other things, help correctional officers keep track of the prisoners' whereabouts. Likewise, in schools and day care centers students may wear body monitors. Hospital patients may wear body monitors to sense various patient parameters. Body monitors may be placed on various items of industrial equipment to sense operational parameters. In these and other situations, some form of control facility receives or otherwise processes information from the body monitors. For security reasons and/or for judging the validity of information being received from the body monitors, the control facility may need to verify that the body monitor is actually associated with the body being monitored.
Techniques are known for detecting the absence of a body. These known techniques may demonstrate acceptable reliability when body monitors are used in stable environments or with cooperative people. In situations where body monitors are used around uncooperative or otherwise mischief-prone people, or when the environment within which body monitors are used changes widely or is tamper-prone, reliably detecting the absence of a body is a difficult but important task.
One known body detector intended for use in connection with prisoner monitoring senses the capacitance of a capacitor that uses the body as a dielectric. An alarm is annunciated whenever the capacitance drops below a predetermined minimum capacitance. No alarm is declared so long as the capacitance remains above the minimum capacitance. The predetermined minimum capacitance is set at a level slightly greater than the value obtained when air is the only dielectric of the capacitor.
Failures in this and other alarm systems may result from either of two error conditions. One error condition is the "false alarm" and the other is the "false silence." The false alarm error occurs when the alarm system declares an alarm in response to a situation where no alarm should have been declared. The false silence error occurs when the alarm system fails to declare an alarm in response to a situation where an alarm should have been declared.
When a body detector uses the above-discussed technique for comparing a body capacitance against a minimum capacitance, the minimum capacitance may be set very low to minimize false alarms. However, the capacitances being detected for body monitoring are very low, and any minimum capacitance that significantly reduces false alarms is difficult to distinguish from stray capacitance. In other words, false alarm failures can be reduced only at the expense of false silence failures. Consequently, the system is unreliable because the system experiences either numerous false alarm errors or numerous false silence errors.